A variety of electrical circuits have been developed for preventing or detecting hazardous, or potentially hazardous, conditions. One potentially hazardous condition in which such a circuit is highly desirable relates to the storage of flammable materials. Flammable materials such as solvents are frequently stored in metal drums or other containers which are subject to electrostatic charge buildup. If a sufficient electrostatic charge is stored by the metal container, the flammable material stored in the container can ignite. To prevent this from occurring, it is desirable to provide the metal containers with a grounding circuit which can indicate whether the metal container is grounded or is susceptible to an electrostatic charge. Proper connection of the circuit is critical, and can be made difficult by paint, dirt, rust or other coatings which may be present on the surface of the metal container.
Commercial systems are available to monitor ground connections. For example, a system is available from Special Technical Services of Allentown, New Jersey which includes a ground clamp with an insulated contact, a processor, and a display panel. There is one processor for each clamp and container to be grounded. The processor is powered by a 117 volt AC power source and the system requires the services of an electrician to hard-wire the system using relatively expensive explosion-proof wiring. The present cost for each unit is approximately $1000, plus hard wire installation costs of approximately $1000; therefore, the present cost of providing detection devices can be approximately $2000 per container. Assuming a facility having 250 metal containers for storing flammable materials, the ground detection costs using this system can be approximately $500,000 per facility. It would be desirable for a ground detection device and system to be simple and inexpensive.
The commercial systems described above typically are not capable of indicating whether a connection is loose (i.e., subject to disconnections of, for example, 1 microsecond or more). Because a detection device which has a loose connection with a conductive material cannot be relied upon to correctly indicate whether the material is grounded, it would be desirable for a ground detection device to also indicate whether its connection to the material is loose.
Further, the commercial systems described above are fixed, since the hard wiring renders the circuitry non-portable. The indicator boxes in a conventional system also tend to be relatively large and heavy. Because these conventional systems are powered by AC wiring, they are inoperative during power failures. Conventional systems typically also use filament lamps for indicators, which are subject to burn out.
Accordingly, it would be desirable for a ground and bond detection device to be practical, low-cost, simple, reliable, portable, and intrinsically safe.